In a spread spectrum communication system, downlink transmissions from a base station to a mobile station include a pilot channel and a plurality of traffic channels. The pilot channel is decoded by all users. Each traffic channel is intended for decoding by a single user. Therefore, each traffic channel is encoded using a code known by both the base station and mobile station. The pilot channel is encoded using a code known by the base station and all mobile stations. Encoding the pilot and traffic channels spreads the spectrum of transmissions in the system.
One example of a spread spectrum communication system is a cellular radiotelephone system according to Telecommunications Industry Association/Electronic Industry Association (TIA/EIA) Interim Standard IS-95, "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System" ("IS-95"). Individual users in the system use the same frequency but are distinguishable from each other through the use of individual spreading codes. Other spread spectrum systems include radiotelephone systems operating at 1900 MHz, commonly referred to as DCS1900. Other radio and radiotelephone systems use spread spectrum techniques as well.
IS-95 is an example of a direct sequence code division multiple access (DS-CDMA) communication system. In a DS-CDMA system, transmissions are spread by a pseudorandom noise (PN) code. Data is spread by chips, where the chip is the spread spectrum minimal-duration keying element. One system parameter is the chip duration or chip time. In an IS-95 system, the chip clock rate is 1.2288 Mega-chips per second, equivalent to a chip time of about 0.814 .mu.sec/chip.
Mobile stations for use in spread spectrum communication systems commonly employ RAKE receivers. A RAKE receiver includes two or more receiver fingers which independently receive radio frequency (RF) signals. Each finger estimates channel gain and phase using the pilot signal. The gain and phase estimates are used to coherently demodulate the RF signal, thereby producing an estimate of each traffic symbol. The traffic symbol estimates of the individual receiver fingers are combined in a symbol combiner to produce a better estimate of the traffic symbols.
A RAKE receiver is used in spread spectrum communication systems to combine multipath rays and thereby exploit channel diversity. Multipath rays include both line of sight rays received directly from the transmitter as well as rays reflected from objects and terrain. The multipath rays received at the receiver are separated in time. The time separation or time difference is typically on the order of several chip times. By combining the separate RAKE finger outputs, the RAKE receiver achieves path diversity.
Generally, the RAKE receiver fingers are assigned to the strongest set of multipath rays. That is, the receiver locates local maxima of the channel impulse response. A first finger is assigned to receive the strongest signal, a second finger is assigned to receive the next strongest signal, and so on. As the channel changes, due to fading and other causes, the finger assignments are changed. After finger assignment, the time locations of the local maxima drift slowly, and these changes are tracked by time tracking circuits in each finger.
One limitation on the performance of a DS-CDMA receiver is multiple-access interference at the receiver. Considered on a per finger basis, there generally are two sources of multiple-access interference on the forward link, from base station to the subscriber unit. The first source is multipath originating from the same base station or the same sector of the same base station as the received signal of interest. The multiple traffic signals transmitted from the base station are orthogonal at the base station's transmitter, because the covering Walsh codes are orthogonal. In the RAKE receiver, interference from orthogonal received traffic signals is completely suppressed. However, multipath in the channel between the base station and the receiver destroys the orthogonality of the Walsh codes by introducing time delay. As a result, some multiple-access interference is introduced. For a finger assigned to a signal from one sector, the second source of multiple-access interference is interference from sectors other than that to which the finger is assigned, both those sectors in soft-handoff with the subscriber unit and those not in soft-handoff with the subscriber unit. The signals transmitted from these other sectors are not orthogonal, regardless of the channel, so some multiple-access interference is introduced at the receiver.
As indicated, the sources of multiple access interference must be viewed on a finger by finger basis. Different fingers may be assigned to different sectors. Thus, same-sector multiple access interference for finger 1 will be other-sector interference for finger 2, if finger 2 is assigned to a different sector.
Noise suppression systems developed for other types of communication systems do not apply to DS-CDMA systems, particularly as applied in the subscriber unit. In some of these prior systems, the number of chips per symbol for the spreading sequence is equal to the period of the spreading sequence. In other prior systems, interference suppression is applied at the base.
In DS-CDMA systems, the spreading sequence may be many times longer than the number of chips per symbol. In IS-95, for example, the spreading sequence repeats every 512 symbols. If the period of the spreading sequence is not equal to the number of chips per symbol, interference suppression is much more difficult. The multiple-access interference in such a situation is no longer stationary. Rather, it is time varying. Both the interference and the coefficients of the interference suppression receiver change completely from one symbol to the next. Because the multiple-access interference is time varying, interference suppression cannot be implemented adaptively and the receiver must explicitly estimate certain features of the interference before the interference can be suppressed.
Accordingly, there is a need in the art for an improved interference suppression technique for DS-CDMA systems.